1. Field of the Invention
The present invention relates to a reflective guest-host liquid-crystal display and a method for manufacturing the same. More particularly, the present invention relates to a technique for improving the utility efficiency of the incident light by providing a .lambda./4 phase shifter (quarter-wavelength plate) and a reflective layer within the display. More particularly, the present invention relates to a reflective guest-host liquid-crystal display prepared according to an active matrix structure with built-in switching devices for driving pixels, and a manufacturing method of the liquid-crystal display.
2. Description of the Related Art
FIG. 5 shows a cross-sectional structure of a reflective guest-host liquid-crystal display equipped with a .lambda./4 phase shifter and a reflective layer within the display disclosed in Japanese Patent Laid-Open No. 6-222351. A reflective guest-host liquid-crystal display 101 shown in the figure includes a pair of upper and lower substrates 102 and 103, a guest-host liquid-crystal 104, a dichroic dye 105, and a pair of upper and lower transparent electrodes 106 and 110, a pair of upper and lower alignment layers 107 and 111, a reflective layer 108, and a .lambda./4 phase shifter 109. The pair of upper and lower substrates 102 and 103 are composed of an insulating material such as glass, quartz, or plastic. At least the upper substrate 102 is transparent. The guest-host liquid-crystal 104 containing the dichroic dye 105 is held in a space formed between the pair of substrates 102 and 103. The guest-host liquid-crystal includes nematic liquid-crystal molecules 104a. The dichroic dye 105 is a so-called p-type dye having a transition dipole moment substantially parallel to the major axis of the liquid-crystal molecules 104a. Although not shown in the figure, switching devices are integratedly formed on the inner surface 102a of the upper substrate 102. The transparent electrode 106 is formed into a matrix pattern to form pixel electrodes and driven by the corresponding switching devices. Furthermore, the inner surface 102a of the upper substrate 102 is covered with the alignment layer 107 made of a polyimide resin or the like. The surface of the alignment layer 107 is, for example, rubbed to homogeneously align the nematic liquid-crystal molecules 104a, in other words, the liquid-crystal molecules 104a are aligned such that their major axis is parallel to the substrate surface.
Meanwhile, on the inner surface 103a of the lower substrate 103, a reflective layer 108 made of aluminum, etc. and the .lambda./4 phase shifter 109 composed of a polymer liquid-crystal, etc. are formed in that order. Moreover, the transparent electrode 110 and the alignment layer 111 are provided on the .lambda./4 phase shifter 109 in that order.
Operation for displaying an image in black and white mode in the above-described reflective guest-host liquid-crystal display 101 will be briefly explained below:
The nematic liquid-crystal molecules 104a, as well as the dichroic dye 105, are aligned homogeneously under no applied-voltage. When light coming from the side of the upper substrate 102 enters into the guest-host liquid-crystal 104, the dichroic dye 105 absorbs a light component whose oscillation face is parallel to the major axis of the dichroic dye 105. In addition, a light component whose oscillation face is perpendicular to the major axis of the dichroic dye 105 is transmitted through the guest-host liquid-crystal 104, circularly polarized by the .lambda./4 phase shifter 109 formed on the surface 103a of the lower substrate 103, and reflected by the reflective layer 108. The polarization direction of the light component is thereby reversed, and the reflected light component is retransmitted through the .lambda./4 phase shifter 109 and allowed to have an oscillation face parallel to the major axis of the dichroic dye 105. The polarized light component is absorbed into the dichroic dye 105, resulting in a substantially black image in the display. Meanwhile, under applied voltage, the nematic liquid-crystal molecules 104a, as well as the dichroic dye 105, are aligned perpendicular to the electric field direction. Light entering from the side of the upper substrate 102 is not absorbed into the dichroic dye 105, and is transmitted through the guest-host liquid-crystal layer 104 and reflected by the reflective layer 108 without being polarized by the .lambda./4 phase shifter 109. The reflected light is re-transmitted through the .lambda./4 phase shifter 109 and emerges from the guest-host liquid-crystal layer 104 without being absorbed into the dichroic dye 105, thereby resulting in a white image in the display.
However, according to the above-mentioned structure, switching devices are formed on the incident-side substrate. Since the switching devices are composed of thin-film transistors (hereinafter referred to as "TFTs") and the like, they cut off the incident light. The aperture ratio of pixels thereby disadvantageously decreases. FIG. 6 is a fragmentary sectional diagram showing a structure of a liquid-crystal display which is under development for overcoming the above problem of decreased aperture ratio and which has not yet been published as a prior art. In this liquid-crystal display, switching devices are integratedly formed on a reflection-side substrate. As is shown in the figure, a transparent counter electrode 203a is formed on the entire surface of an upper substrate 201, and a pixel electrode 204a is provided on a lower substrate 202 by dividing a reflective electrode into small portions according to a matrix shape. That is, this liquid-crystal display is an active matrix type. In addition to the pixel electrode 204a formed into a matrix pattern, a TFT corresponding to the individual pixel electrode 204a is integratedly formed on the inner surface of the substrate 202. The TFT is used as a switching device for driving the individual pixel electrode 204a. In other words, the "on/off" state of each TFT is selectively controlled to write a signal voltage in the corresponding pixel electrode 204a. A drain region D of the TFT connects to the pixel electrode 204a and a source region S connects to a signal line 221. A gate electrode G of the TFT connects to a gate line. Moreover, a storage capacitor Cs is formed corresponding to the pixel electrode 204a. By a planarization layer 222, the pixel electrode 204 is electrically separated from the TFT, the storage capacitor Cs, the signal line 221, and the like. Meanwhile, the transparent electrode 203a is formed on the entire inner surface of the upper substrate 201. The substrates 201 and 202 are in an opposed position at a predetermined distance while holding an electro-optical substance 205 therebetween. The electro-optical substance 205 has a laminated structure including a guest-host liquid-crystal 205 and a .lambda./4 phase shifter 207. The guest-host liquid-crystal 206 contains nematic liquid-crystal molecules 209 and a dichroic dye 208, and is homogeneously aligned by an upper alignment layer 210 and a lower alignment layer 211. The .lambda./4 phase shifter 207 is formed as a film along the pixel electrodes 204a.
When a signal voltage is written in the pixel electrode 204a, an electric field is generated between the pixel electrode 204a and the counter electrode 203a facing the pixel electrode 204a. The guest-host liquid-crystal 206 changes between the absorption state and the transmission state. Since such optical change occurs in each of the pixel electrodes 204, the desired image can be achieved in the display. The TFT, the storage capacitor Cs, the signal line 221, and the like are positioned under each of the pixel electrode 204. These components do not affect the pixel aperture rate because they do not stand in the incident-light path. In other words, the actual area of each pixel electrode 204a can be utilized as the pixel aperture, resulting in a bright image in the display.
However, in addition to the guest-host liquid-crystal 206, a plurality of layers are provided between the pixel electrode 204a and the counter electrode 203a in a structure shown in FIG. 6. Among such layers, the .lambda./4 phase shifter 207 is particularly thick. When a voltage is applied to the pixel electrode 204a, the polarization charge is concentrated in each interface between the above layers. Thus, a reverse electric field is generated in each interface, resulting in deteriorated image quality such as decreased contrast and after-image. Moreover, the effective voltage applied to the guest-host liquid-crystal 206 is disadvantageously reduced by providing a .lambda./4 phase shifter 207 on the pixel electrode 204a.